In precision work such as die making or other operations requiring close tolerances, it is important that the workpiece be held firmly against the bed of the vise. It has been found, however, that there is a tendency for the workpiece to move upwardly a slight amount from the bed, not only when the jaws are tightened, but during the actual working operation.
There are many reasons why this occurs, however the greatest contributor is wear. A vise which is worn, particularly along the bed rail or key way on which the movable jaw slides, permits a few thousandths of an inch of upward movement of the movable jaw relative to the bed which results in unwanted movement of the workpiece relative to the tool. Whether this occurs during the initial application of the workgripping force or during the actual work operation itself is immaterial since in both instances the inaccurate positioning of the workpiece relative to the tool can cause the workpiece to be ruined.
The conventional solution has been to strike the workpiece with a hammer both when tightening the vise or after the workpiece has crept upwardly. This seldom solves the problem. There is no guarantee that the workpiece won't creep up again, and striking an intricate or delicate workpiece even with a soft-head hammer can damage it. Furthermore, there is no guarantee that the workpiece will return to its original position relative to the tool. This can result in a different depth of cut.
Numerous attempts have been made to rectify the problem mechanically. For example, U.S. Pat. No. 3,791,640 to Clugage discloses flexible workpiece hold-down elements for use in conjunction with the jaws of a vise. The elements are positioned to engage opposite sides of a workpiece while being compressed between the stationary and movable jaws. The hold-down members flex internally and in so doing, because of their shape, apply a force to the workpiece in a downward direction toward the vise bed. However, the engagement between the hold-down elements and the workpiece is linear. If the workpiece is struck inadvertently or if the cutting force on the workpiece is too great, it is possible for the workpiece to fly out of the vise, which can not only result in damage to the workpiece, but cause potential injury to a machinist standing close by.
U.S. Pat. No. 2,861,492 to Hokanson discloses vise jaw plates in the form of blocks which have flat surfaces for engaging a workpiece. When the jaws are tightened, the blocks move, not only to grip the workpiece, but to urge it downwardly toward the vise bed. The blocks are mounted on the jaws in order to be able to twist or pivot to accommodate a workpiece which is irregular in shape, particularly one which is wider at one end than the other. This produces an imprecise force on the workpiece and will not per se guarantee the desired positive bedding force for which the vise jaws are intended.
The two above identified patents are typical of many which are intended to solve the problem but which have inherent limitations generally of the types described.